Sudden cardiac death (SCD) and syncope are frequent complications in patients with hypertrophic cardiomyopathy (HCM). We have previously shown that in HCM patients, a strong association exists between inducibility of ventricular tachycardia (VT) at electrophysiologic (EP) studies and a history of sudden cardiac arrest (SCA) and syncope. To obtain a better understanding of the mechanisms responsible for SCA and syncope in HCM patients, we performed detailed hemodynamic and EP assessment of sudden cardiac arrest survivors (N=30) and HCM patients with syncope (N=55). We also assessed the utility of amiodarone (N=32) and of the implantable defibrillator (AICD, N=25) to prevent SCD, and role of dual chamber (DDD) pacemakers to relieve symptoms due to left ventricular obstruction (N=10). Five mechanisms explained SCA in all the HCM patients presented with this event: 1) VT, 2) severe obstruction to the outflow of blood in the left ventricle, 3) myocardial ischemia induced by moderate increases in heart rate (previously unrecognized syndrome affecting young HCM patients), 4) an atrial tachycardia resulting in severe hypotension, and 5) bradycardia. Factors that were associated with syncope were 1) significant obstruction and inducibility of VT at EP studies. Two variables independently predicted subsequent SCO: 1) a history of previous SCA and 2) inducibility of VT at EP studies. Empiric amiodarone therapy, reported to prevent SCD in HCM patients with VT Holter, was associated with SCD in 8 patients within 3 months of initiation of drug therapy - nearly all in patients with VT on Holter. EP studies showed that amiodarone frequently aggravated conduction problems and in most HCM patients was proarrhythmic. Notably, symptomatic patients in whom amiodarone facilitated VT induction were more likely to experience SCD subsequently. The AICD device, significantly reduced subsequent SCD (mean follow-up period; 14 months). DDD pacemakers reduced obstruction and resulted in symptomatic improvement in 8/10 patients who would otherwise have needed cardiac surgery.